The gas inside seems to degrade, even when unused, although there’s no way to predict how long any given tube will last.
You can test the tube to verify it’s resonating in the correct TEM00 mode and testing before and after it conks out would be informative. A previous discussion goes over the details:
Components in the high voltage power supply can also fade out. The problem you describe could be due to a supply on the edge of exhaustion.
Because the HV supply is much cheaper and easier to replace than the tube, I’d start by replacing it.
If a new HV supply doesn’t improve the situation, then it’s time for a hard look at the tube.